This invention relates to print cartridges for
printers and more particularly to a print cartridge with an improved aperture geometry.
Electrostatic printers receive images in the form of electronically coded information and convert it to an output on a medium such as paper. Typically an electrostatic printer uses a print cartridge with a plurality of discharge sites which can be controlled to place electrostatically charged particles on a charge receiving surface such as a revolving print drum or moving belt to form charged dots which in turn make up a latent image. Typically, the receiving surface is comprised of an electrically conductive substrate coated with a dielectric coating to enable it to hold charged particles generated by the print cartridge.
In the following description, the charge receiving surface will be described for convenience primarily with reference to a drum.
Once the latent image has been formed on the drum, toner is applied to the image and subsequently transferred to the paper and fused at a nip between the receiving surface and a fusing roller. Advantageously, the print drum and fusing roller revolve on axes which subtend an angle of approximately forty-five minutes to aid in fusing the toned image to the paper.
Excess toner is removed from the drum by a scraper and any remaining latent image is then discharged by an erase head before the drum completes a revolution and starts the printing cycle again.
The electrostatic printer offers many advantages including relatively high-speed printing of computer generated images and the flexibility to print additional copies or to select either portrait or landscape images.
One type of print cartridge which has formed the basis for many modern electrostatic printers is described in U.S. Pat. No. 4,155,093 to Fotland et al which issued May 15, 1979. This type of cartridge provides for the generation of charged particles by an electrical gas breakdown in a field between two electrodes separated by a dielectric substrate. Rows of parallel and equally spaced driver electrodes are attached to one side of the substrate and run from one end of the cartridge to the other. Parallel and equally spaced finger electrodes are located on the opposite side of the substrate and extend diagonally across the driver electrodes. The finger electrodes define discharge sites in the form of a matrix of apertures corresponding to the points where driver and finger electrodes cross. An AC voltage may be applied to the electrodes to cause gas breakdown and charged particle production at edge structures associated with the apertures.
An improvement on this cartridge structure is described in U.S. Pat. No. 4,160,257 to Carrish which issued July 3, 1979. This patent teaches the use of a third or screen electrode separated from the finger electrodes by a dielectric layer. The screen electrode and dielectric layer both have a matrix of openings in alignment with the apertures in the finger electrodes. A DC field can be applied to this electrode to provide a lensing action for focusing charged particles generated by the cartridge to produce more precise charged dots on the printing drum.
Electrostatic print cartridges of the type just described are well suited for producing both text and graphics although they do have limitations when producing large filled areas. In particular this type of cartridge tends to produce a series of very small bumps along the leading and trailing edges (with respect to drum rotation) of filled areas. These bumps occur with a spacing equal to the finger electrode spacing and are more pronounced on the trailing edges of images.
In the cartridge structure described above, the apertures in the matrix are arranged in a series of diagonal rows coinciding with the finger electrodes. The apertures in each diagonal row are arranged to produce charged dots on a particular segment of the drum. Charged particle production can be initiated at each aperture as needed to place a charge on a corresponding point on the drum as the drum rotates past the aperture. The arrangement of apertures is such that it is possible to place charged dots anywhere within a selected zone on the circumference of the drum as it rotates past the cartridge to build up any selected image.
The production of print "bumps" by prior art cartridges will now be explained with reference to the placement of charged dots on a discharged drum to produce a latent image on the drum. The dots will oe laid down as the section of the receiving surface to be printed passes the print cartridge with each finger electrode serving to provide dots in a corresponding segment of the drum surface.
Initially, the first dot in each of the segments is placed by the first aperture in each of the rows of apertures. These initial dots are therefore separated from one another by a distance determined by the length of the segment of image to be created by each of the finger electrodes and this in turn is a function of the number of driver lines used in the cartridge. Because these dots are spaced apart, they have no effect on one another. The rotating drum will then advance to a position where the next aperture of each electrode is available to create a dot. These new dots will be placed immediately adjacent the initial dots in their respective image segments. The shape and position of the new dots will be affected by the charge carried by the initial dots and will tend to displace the new dots away from the initial dots.
Subsequent dots are laid down in sequence adjacent the previously created dots causing the segments to grow and the influence of the charges on the previous dots will become more significant as the image grows so that the dots will be shifted away from the desired position.
The last dot in each segment is an exception and will be laid down between the second last dot in that segment and the first dot in the adjacent segment. Consequently it will be squeezed between these dots by the effect of repulsion caused by like charges and forced to overlap above and below the rest of the adjacent dots. This new dot then projects out of alignment of the other dots to create what will be referred to as a "bump". The bump above the line will be referred to as the leading edge bump, and at the end of the image, there will be a trailing edge bump and the creation of this trailing edge bump will be described.
Production of a second line in the image following the first filled line will now be discussed. Because all of the dots in the second line will be located adjacent the previously laid down dots in the first line, the second line will be shifted downwards. The placement of each dot after the first dot in each segment will be affected by both the previously laid down dots in that line and the dots in the line above.
In a similar manner to the preceding case the last dot in each segment must fit between the second last dot in that segment and the first dot in the next segment. This dot will also be repelled by the last dot in the corresponding segment on the previous line. Thus the dot will be surrounded on three sides by like charges and will be forced to overlap beneath the other dots in that line due to the repulsion of like charges. This is the case of the trailing edge bump which projects further from the surrounding dots than a leading edge bump.
Print bumps are not terribly significant for many uses such as producing text and forms. However, for some uses such as the production of high definition graphics, print bumps stand out from the surrounding lines and are undesirable.